It is well known that the strongest, most uniform fiber-reinforced resin structural members are obtained by molding lay-ups of sheets of long reinforcing fibers which have been impregnated with all of the resin necessary to provide a void-free product. Because no resin need be added during the molding process, such sheets are called "prepregs" and inherently provide structures of higher strength and better uniformity than can be achieved where the molder must apply resin to the reinforcing fibers.
Prepreg sheets of high-melting thermoplastic resins tend to be stiff and slippery at room temperature and hence difficult to lay up except in flat molds. A residue of approximately 10-15% by weight of solvent in the resin can provide sufficient pliability and tackiness for use in contoured molds, but the solvent must be completely evaporated prior to molding to insure strong, void-free structures. If all of the solvent is driven off before the prepreg sheet is put to use, it is customary to preheat the sheet to give it some flexibility and surface tack.
Due to such difficulties, prepreg sheets are more commonly based on thermosetting resins of initially low molecular weight. Admixtures of such resins with crosslinking agents tend to be supple and tacky at room temperature. However, thermosetting resin compositions which have high softening temperatures have seldom been used to make prepreg sheets.